The present invention relates to a memory employing a magnetic thin film, and to a recording/reproduction method for such a memory. More particularly, it relates to a magnetic thin film memory adapted to record information on the basis of the direction of magnetization, and to a recording/reproduction method therefor.
FIG. 18 an explanatory view schematically illustrating a conventional magnetic thin film memory in assembled condition which is disclosed in "Magnetic Thin Film Technology," Electrical Engineering Lectures Vol. 5, MARUZEN Co., 1977, p. 254.
To be described first is an example of a process for manufacturing such a conventional magnetic thin film memory. A flat smooth glass substrate is closely covered with a mask having rectangular apertures. A Fe-Ni alloy film is then formed to have a thickness of about 2000 .ANG. on the thus masked substrate in a vacuum evaporator. Thus, a multiplicity of memory elements MF are provided in a matrix configuration at a time. Wires for driving the memory elements are formed by forming copper wires on both sides of a thin plate of epoxy resin or a polyester sheet by photoetching technology so as to cross each other at right angles. The wires on respective side of the plate or sheet are word lines and bit lines, respectively. The memory elements and the wires are assembled together by pressing the wires against the memory elements so that the cross point of the wires are located on each of the memory elements.
To be described next is the principle of operations of the conventional magnetic thin film memory. In FIG. 18, wires W.sub.1 to W.sub.3 disposed parallel to the easy magnetic axis are word lines, while wires D.sub.1 to D.sub.3 crossing the wires W.sub.1 to W.sub.3 at right angles are digit lines. Each of the digit lines D.sub.1 to D.sub.3 also functions as a sense line for reading the state of the bit stored.
Arrows C and E, respectively, indicate the directions of magnetization of magnetic thin films which correspond to the respective states of the bit stored. The arrow C upwardly orienting in the drawing herein means that information "0" is recorded, while the arrow E downwardly orienting means that information "1" is recorded. Further, magnetic fields Hd and Hw, herein, are generated by a digit current Id and a word current Iw, respectively, and exerted on the magnetic thin films. When the word current Iw, which is of single-polarity pulse, is made to flow to the word line W.sub.1 selectively, all the memory elements under the word line W.sub.1 are provided with the magnetic field Hw and, as a result, the direction of magnetization thereof orients along the hard magnetic axis. Depending on whether the magnetization is rotated from the state for "1" or "0", pulse voltages different in polarity are induced in respective digit lines. These pulse voltages function as readout voltage. When information is to be recorded, the current Id is made to flow at the same time when pulses of the current Iw fall, whereby the magnetic field Hd corresponding to an information signal is generated on the magnetic thin film under the digit line of which magnetization is aligned along the hard magnetic axis. The direction of magnetization is determined by the composite vector of both magnetic fields Hw and Hd, so that information of "1" or "0" can be recorded in each of the memory elements. The current Iw is a value of an electrical current such as to generate the magnetic field Hw capable of rotating the magnetization on the magnetic thin film from the easy magnetic axis to the hard magnetic axis. The current Id is a value of an electrical current such as to generate the magnetic field Hw which is about a half of the coercive force Hc of the magnetic thin film.
FIG. 19(a) schematically shows the structure of another prior art magnetic thin film memory which is disclosed in, for example, IEEE TRANSACTIONS ON MAGNETICS, Vol. 24, No. 6, 1988, pp. 3117 to 3119. FIG. 19(b) is a fragmentary section showing one element portion of the memory shown in FIG. 19(a). In FIG. 19(b) numeral 1 denotes an MR(magneto-resistive) layer, numeral 2 denotes a sense line, numeral denotes 3 a word line, and numeral 4 denotes an insulating layer.
Binary information ("0" or "1") is recorded as the direction of magnetization (upward or downward in the drawing). The MR layer 1 is composed of Fe and Ni together with a faint amount of Co and designed so that the easy magnetic axis thereof would extend vertically in the drawing.
To be specifically described is the process of recording to such a magnetic thin film memory.
When a random access recording is to be performed to, for example, a memory element 111 in FIG. 19(a), current is made to flow in a sense line 21 and a word line 31 which pass through the memory element 111, selectively among the sense lines and word lines. The recording sense current flowing through the sense line 21 produces at the memory element 111 a recording sense magnetic field orienting upward or downward in the drawing. The direction of the recording sense magnetic field, upward or downward, is determined by the direction of the recording sense current, leftward or rightward in the drawing. On the other hand, the recording word current flowing through the word line 31 produces at the memory element 111 a recording word magnetic field orienting leftward or rightward. Unlike the sense current, the word current may be made to flow in one direction. For example, it is made to flow in a direction such that the recording word magnetic field orients rightward. Thus, the recording sense magnetic field and the recording word magnetic field are applied to the memory element 111. A change in the magnetization state of the memory element 111 in this recording operation is shown in FIG. 20.
FIG. 20(a) shows the magnetization state of the MR layer before application of magnetic field. The magnetization state, whether upward or downward, before application of a magnetic field has nothing to do with the subsequent recording process. If the recording sense magnetic field produced by making current flow in the sense line orients upward, a composite magnetic field composed of recording sense magnetic field 82 and recording word magnetic field 83 orients in the upper-right direction as shown in FIG. 20(b), and the magnetization also orients in the upper-right direction as shown in FIG. 20(c). When the magnetic field is eliminated (i.e. when the current is stopped), the magnetization orients upward along the easy magnetic axis as shown in FIG. 20(d), hence, assumes a stable condition. On the other hand, if the recording sense magnetic field orients downward, a composite magnetic field 85 composed of recording sense magnetic field 82 and recording word magnetic field 83 orients in the lower-right direction as shown in FIG. 20(e), and the magnetization also orients in the same direction as shown in FIG. 20(f). When the magnetic field is eliminated, the magnetization orients downward as shown in FIG. 20(g) and assumes a stable condition. Thus, changing the direction of the recording sense current enables recording in the direction, upward or downward. In FIG. 19, a magnetic field is also applied to memory elements 112, 113 . . . , 121, 131 . . . as well as the memory element 111. However, these memory elements other than the memory element 111 are applied with just either the recording sense magnetic field or the recording word magnetic field. This is insufficient to reverse the magnetization, hence, the initial recording state is retained. From the reversed point of view, values of the recording sense current and recording word current need to be selected so that the magnetization of only the memory element 111 would be reversed. The above-mentioned is the principle of recording.
The principle of reproduction is as follows:
When a random access reproduction is to be performed on, for example, the memory element 111 in FIG. 19, current is made to flow in the sense line 21 and word line 31. At this time, the reproduction sense current flowing through the sense line 21 and the reproduction word current flowing through the word line 31 produces at the memory element 111 a reproduction sense magnetic field and a reproduction word magnetic field, respectively. These reproduction magnetic fields are set smaller than the recording magnetic fields. Therefore, recorded information will never be destroyed.
The reproduction operation by these magnetic fields is described with reference to FIG. 21. As shown in FIG. 21(b), the direction of current is predetermined so that a reproduction sense magnetic field 82 would orient upward and a reproduction word magnetic field 83 rightward. Hence, a composite magnetic field 85 orients in the upper-right direction. Under the influence of the composite magnetic field 85, the upward-oriented magnetization of an upward record (FIG. 21(a)) is slightly inclined to the upper right (FIG. 21(c)). On the other side, the downward-oriented magnetization of a downward record (FIG. 21(e)) is inclined at a larger angle (FIG. 21(f)). When the magnetic field is eliminated, the magnetization in either case resumes its original recording state (FIGS. 21(d) and 21(g)).
Referring to FIG. 22, let the angle formed by the direction of the sense current and the direction of the magnetization be .phi., then the resistance R due to the so-called "anisotropic MR effect" is given by the equation: EQU R=R.sub.o (1+.DELTA.cos.sup.2 .phi.)
where R.sub.o is a resistance when the direction of magnetization is parallel, and .DELTA. is a MR coefficient which is determined by the material. Accordingly, when the resistance between the opposite ends of the sense line is measured, in the case of the recording state of FIG. 21(c), just a small change in the resistance occurs because the angle determined by the direction of the magnetization with respect to that of the sense current is large. In the case of the recording state of FIG. 21(f), on the other hand, a large change in the resistance occurs because the angle determined by the direction of the magnetization with respect to that of the sense current is small. In practice, since the reproduction current is constant and the resistance between the opposite ends of the sense line is proportional to the voltage therebetween, the voltage is measured for reproduction.
As can be understood from the above, a small change in voltage occurs in the case of upward magnetization, whereas a large change in voltage occurs in the case of downward magnetization. Although a plurality of memory elements are connected in series on the sense line in FIG. 19, memory elements other than the memory element 111 are not applied with the reproduction word magnetic field. Hence, they are free of change in resistance and will not contribute to reproduction. Accordingly, the information written in the memory element 111 only is selectively reproduced. The principle of reproduction is as above.
The prior art magnetic thin film memories utilize, in reading, an anisotropic MR effect such that the resistance changes depending on the angle of the magnetization direction of a MR layer with respect to the direction of current or a very small electromagnetic induction voltage produced by rotation of the magnetization. For this reason, the rate of change in resistance is very small, or as small as about 0.5% and, hence, the SN ratio must be improved by carrying out an averaging treatment for several microseconds, so as to secure a sufficient SN ratio for reproduction. The averaging treatment for such a long time causes the access time for reproduction to be lengthened and the data transfer rate to be degraded, raising a problem of limited use of the memory.
Further, utilizing an electromagnetic induction in reading, the magnetic thin film needs to be large enough in size because the electromagnetic induction voltage is proportional to the magnitude of magnetic moment. For this reason, there arises another problem that it is impossible to increase the amount for recording information per unit area of the magnetic thin film.
An object of the present invention is to provide a magnetic thin film memory which offers a remarkably improved SN ratio, largely shortened access time and greatly enhanced data transfer rate while performing an increased recording capacity per unit area, and to provide a recording/reproduction method therefor.
To overcome the foregoing problems, it was formerly proposed that a magnetic thin film memory element employing a method wherein information is recorded on the basis of the direction of the magnetization of a magnetic thin film and the recorded information is read out utilizing a change in resistance of the magnetic thin film due to a magnetoresistive effect, the magnetic thin film comprising a magnetic layer a having a large coercive force, a magnetic layer b having a small coercive force, and a nonmagnetic layer c, the layers a, b and c being stacked in the sequence of a/c/b/c/a/c/b/c . . . with the layer c interposed between the layers a and b and vice versa. In addition, there was proposed a magnetic thin film memory element employing a method wherein information is recorded on the basis of the direction of magnetization of a magnetic thin film and the recorded information is read out utilizing a change in resistance of the magnetic thin film due to a magnetoresistive effect, the magnetic thin film comprising a magnetic layer a having a large coercive force, a magnetic layer b having a small coercive force, and a nonmagnetic layer c, the layers a, b and c being stacked in the sequence of a/c/b/c/a/c/b/c/ . . . with the layer c interposed between the layers a and b and vice versa, wherein information is recorded on the basis of the direction of magnetization of the magnetic layer b (refer to Japanese Patent Application No. 63028/1992).
The above memory elements were confirmed to have such an effect that a sufficiently large read signal can be obtained even if the memory element is reduced in size.